Briefly, protrusions commonly termed hillocks are known to develop on the surface of thin metallic films which are subjected to thermal cycling between certain temperature extremes. Such thermal cycling is often either necessary for fabrication of the thin films or the structure of which it forms a part, or during operation of the film or structure in its intended use, or during excursion from storage temperature or from use temperature. The protrusions or hillocks are detrimental to fabrication of devices using such films and to the devices once fabricated usually because there are associated with the thin metallic films thin and fragile layers, for example, insulating layers, which are damaged by the development or growth of the hillocks; also, hillocks can affect the intrinsic device properties of a single film, for example, in magnetic devices
For greater detail, a thin film which is deposited on a substrate is stressed because of differential thermal expansion of the film and the substrate. In most metal and substrate composite structures the metal has a larger coefficient of thermal expansion than that of the substrate and as a result biaxial compressive stress is set up in the metal film when the film and substrate composite is heated from one temperature to a higher one. One of the ways in which a film tends to relieve the induced compressive stress is by a growth of small protrusions or hillocks on the film surface.
The presence of hillocks on the surface of a thin metallic film has been detrimental to both the fabrication of a film itself and to devices in which it is introduced. In many multilayer devices utilizing a thin metallic film there are present thin and fragile insulator (or protective) layers which are either impaired or effectively destroyed by hillock formation on the metal film during fabrication or use of the device.
In the prior art thin metallic films are known in which alloying additions are incorporated for several metallurgical purposes. Illustratively, copending patent application Ser. No. 791,371 and commonly assigned utilizes copper additions in thin A1 conductive films to impart resistance to the film against damage consequent from current induced transport phenomena, e.g., electromigration; and U.S. Pat. No. 3,427,154 issued Feb. 11, 1969, disclosed a procedure for obtaining amorphous alloys in film form wherein codeposition of the components obtains an amorphous film which is metastable to relatively high temperature. However, the prior art is not known to have provided sufficient understanding of the mechanisms by which hillock formation produced on the surface of metallic film to have alleviated generally the troublesome difficulty for fabrication of films and associated devices wherein hillock growths have detrimental consequences.
Included among the types of multilayer devices which are detrimentally affected by hillock formation on metallic films therein are magnetic film devices, superconductor tunneling devices, and semiconducting devices, particularly those with multilevel interconnections. In the magnetic type of device, conductive metal film is separated from magnetic film by an insulating layer; and thermal cycling during fabrication of the structure with consequent hillock formation causes detrimental shorts between the conductive layer and the magnetic layer. In the tunneling devices a superconductive film on a substrate is separated from another superconductive film by an insulating layer and the operation of the device is dependent upon the structural integrity of the insulating layer which is detrimentally impaired by hillock formation on the interface surface between the one film established on the substrate for the device and the insulating layer. In the semiconducting devices, the device is often protected by a glass layer which can be ruptured by hillock formation on the underlying metal conducting films; and, in the case of multilevel interconnection integrated circuits, hillocks may cause electrical shorts between superposed metallic conducting layers.
More particularly, thin film tunneling devices are described in the illustrative literature references of: "The Tunneling Cryotron -- A Superconductive Logic Element Based on Electron Tunneling," by J. Matisoo, appearing in Proceedings of IEEE, Vol. 55, No. 2, February 1967 at pages 172-180. This same article also described a Josephson current device which operates on the principle discussed by B. D. Josephson in Phys. Letters, Vol. 1, pages 251-253, July 1962 -- "Possible New Effects in Superconducting Tunneling".
Conventional thin film tunneling devices have numerous problems associated with them. One of the most troublesome of these results from thermal cycling between a low temperature state and room temperature. This cycling may cause stress induced structural changes of electrode material resulting in hillock formation, across the tunnel junction, and thus a short circuit. This problem is especially acute with materials, such as lead, tin, and indium, and other low melting point mmetals when cycled between a superconductive temperature and room temperature. Such cycling of temperature occurs when leads are being deposited on the devices or when there is a failure of the refrigeration system which is used to create an operating environment for the devices. Cycling also occurs when the devices are being stored between usage or are being repaired. In Josephson type devices, where the tunnel barrier is of such small thickness, i.e., 2-20A, the recrystallization problem becomes extremely sensitive, since arrays of these devices are destroyed if there is even minor structural changes in discrete devices in the array. Due to the thinness of the barrier, shorted junctions easily develop from almost any degree of hillock growth.